Today I wanted to re-install WSUS again in a production environment where ConfigMgr 2012 was also on place.
Normally these steps are followed:
1) Uninstall WSUS (without
database, logfiles, updates)
2) Install WSUS again + KB2720211 + KB2734608
3) Reboot the server (you never know)
4) Start "Synchronize
This time I couldn't connect to the WSUS database anymore. Error message: "The existing database is not compatible with this version of Windows Server Update Services 3.0 SP2. Select another database instance or create a new database"
Looking on the existing WSUS Database I found out that permissions were not correct. After adding the Installation account to the WSUS database with Sysadmin permissions it was working again.
Hope it helps!